Since when I moved to WP8 I used CleverToDo.
It's useful since it has the possibility to sync all the tasks online, you just need to create a free account on their site. Plus it's possible to access all your todo from any browser and then you can edit and create elements. The sync service is free, at least for now + I contacted the devs with some question and they had a good support: fast and nice.

Is there any TODO app which has both a metro/desktop interface and a WP8 so that I can sync between the two? I used Wunderlist before but it stopped syncing after they upgraded to version 2 and also it is not available in the WP store any more. So I am looking for a substitute.

And if you would suggest OneNote, then please also tell me how one can add due dates for tasks, because I just couldn't figure it out :(

I really wish that Trello would develop a windows phone app. I like their website, and I used to use the iPhone app a lot. Its great. Really want them to build for Windows phone. They have an app for Windows 8 already.